The invention relates to an electric motor with permanent-magnet excitation, particularly to inside-rotor and outside-rotor motors, comprising motor sections which are movable relative to one another, of which the first motor section forms a multi-pole excitation field in an air gap by means of permanent magnets and of which the second motor section is a soft-iron yoke having pole teeth which project towards the air gap and which carry excitation coils.
Such motors are known as rotation motors, for example from DE 28 23 208 C2 and from DE 31 22 049 C2. However, a problem of such motors is that their efficiency is low although the multi-pole excitation field in the air gap is formed without losses when the motor is stationary.
If these motors are used as drive motors in battery-operated apparatuses their low efficiency will reduce the total operating time attainable with one set of batteries. Regardless of the type of power supply the heat produced in the motor leads to thermal expansion of parts of the motor itself and parts of the apparatus being driven. This necessitates steps in order to prevent the correct operation of the driven apparatus from being impaired as result of these thermal expansions.
Such motors with permanent-magnet excitation exhibit losses in the form of copper losses in the excitation coils of the second motor section, which coils are arranged on pole teeth projecting from the soft-iron yoke towards the air gap, iron losses in the soft-iron yoke and in the pole teeth projecting towards the air gap, and iron losses in the soft-iron yoke connected to the permanent magnet of the first motor section.
On account of the rotation of the motor the two motor sections move relative to one another, as a result of which the magnetic fields in the soft-iron yokes vary in magnitude and direction. These fluctuations of the magnetic fields produce the iron losses in the soft-iron yokes. The copper .losses in the excitation coils of the second motor section are caused by currents in these coils.
The magnetic fields of varying magnitude in the pole teeth projecting from the soft-iron yoke of the second motor section towards the air gap in conjunction with the currents flowing in the excitation coils produce, in known manner, the torque which is characteristic of the operation of such motors. Therefore, the losses caused by these magnetic fields and currents in the second motor section should be reduced only in such a manner that this does not lead to a reduction of the motor torque. The most important step in this respect is the use of magnetic steel sheets to laminate the soft-iron yoke of the second motor section in a direction perpendicular to the axis of rotation of the motor.
On the other hand, the magnitude and direction of the magnetic fields in the soft-iron yoke connected to the permanent magnet of the first motor section need not vary in order to generate the motor torque.